Previous Issue

Table of Contents

Galaxies, Volume 6, Issue 4 (December 2018)

  • Issues are regarded as officially published after their release is announced to the table of contents alert mailing list.
  • You may sign up for e-mail alerts to receive table of contents of newly released issues.
  • PDF is the official format for papers published in both, html and pdf forms. To view the papers in pdf format, click on the "PDF Full-text" link, and use the free Adobe Readerexternal link to open them.
View options order results:
result details:
Displaying articles 1-37
Export citation of selected articles as:
Open AccessArticle Detection of Periodic Radio Signal from the Blazar J1043+2408
Galaxies 2018, 6(4), 136; https://doi.org/10.3390/galaxies6040136
Received: 14 September 2018 / Revised: 28 November 2018 / Accepted: 30 November 2018 / Published: 6 December 2018
Viewed by 71 | PDF Full-text (559 KB) | HTML Full-text | XML Full-text
Abstract
The search for periodic signals from blazars has become a widely discussed topic in recent years. In the scenario that such periodic changes originate from the innermost regions of blazars, the signals bear imprints of the processes occurring near the central engine, which
[...] Read more.
The search for periodic signals from blazars has become a widely discussed topic in recent years. In the scenario that such periodic changes originate from the innermost regions of blazars, the signals bear imprints of the processes occurring near the central engine, which are mostly inaccessible to our direct view. Such signals provide insights into various aspect of blazar studies, including disk-jet connection, magnetic-field configuration and, more importantly, strong gravity near the supermassive black holes and release of gravitational waves from binary supermassive-black-hole systems. In this work, we report the detection of a periodic signal in the radio light curve of blazar J1043+2408 spanning ∼10.5 years. We performed multiple methods of time-series analysis, namely, epoch folding, Lomb–Scargle periodogram, and discrete autocorrelation function. All three methods consistently revealed a repeating signal with a periodicity of ∼560 days. To robustly account for the red-noise processes usually dominant in the blazar variability and other possible artefacts, a large number of Monte Carlo simulations were performed. This allowed us to estimate high significance (99.9% local and 99.4% global) against possible spurious detection. As possible explanations, we discuss a number of scenarios, including binary supermassive-black-hole systems, Lense–Thirring precession, and jet precession. Full article
Figures

Figure 1

Open AccessReview Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501
Galaxies 2018, 6(4), 135; https://doi.org/10.3390/galaxies6040135
Received: 7 November 2018 / Revised: 3 December 2018 / Accepted: 4 December 2018 / Published: 6 December 2018
Viewed by 73 | PDF Full-text (351 KB) | HTML Full-text | XML Full-text
Abstract
Flux distribution is an important tool to understand the variability processes in active galactic nuclei. We now have available a great deal of observational evidences pointing towards the presence of log-normal components in the high energy light curves, and different models have been
[...] Read more.
Flux distribution is an important tool to understand the variability processes in active galactic nuclei. We now have available a great deal of observational evidences pointing towards the presence of log-normal components in the high energy light curves, and different models have been proposed to explain these data. Here, we collect some of the recent developments on this topic using the well-known blazar Mrk 501 as example of complex and interesting aspects coming from its flux distribution in different energy ranges and at different timescales. The observational data we refer to are those collected in a complementary manner by Fermi-LAT over multiple years, and by the First G-APD Cherenkov Telescope (FACT) telescope and the H.E.S.S. array in correspondence of the bright flare of June 2014. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
Figures

Figure 1

Open AccessArticle Classical and Relativistic Evolution of an Extra-Galactic Jet with Back-Reaction
Galaxies 2018, 6(4), 134; https://doi.org/10.3390/galaxies6040134
Received: 16 October 2018 / Revised: 30 November 2018 / Accepted: 1 December 2018 / Published: 6 December 2018
Viewed by 90 | PDF Full-text (379 KB) | HTML Full-text | XML Full-text
Abstract
We consider a turbulent jet that is moving in a Lane–Emden (n=5) medium. The conserved quantity is the energy flux, which allows finding, to first order, an analytical expression for the velocity and an approximate trajectory. The conservation of
[...] Read more.
We consider a turbulent jet that is moving in a Lane–Emden ( n = 5 ) medium. The conserved quantity is the energy flux, which allows finding, to first order, an analytical expression for the velocity and an approximate trajectory. The conservation of the relativistic flux for the energy allows deriving, to first order, an analytical expression for the velocity, and numerically determining the trajectory. The back-reaction due to the radiative losses for the trajectory is evaluated both in the classical and the relativistic case. Full article
Figures

Figure 1

Open AccessArticle Simulations of the Polarized Sky for the SKA: How to Constrain Intracluster Magnetic Fields
Galaxies 2018, 6(4), 133; https://doi.org/10.3390/galaxies6040133
Received: 30 October 2018 / Revised: 29 November 2018 / Accepted: 29 November 2018 / Published: 4 December 2018
Viewed by 83 | PDF Full-text (1695 KB) | HTML Full-text | XML Full-text
Abstract
The advent of the Square Kilometer Array (SKA) will have unprecedented impact on the study of magnetic fields in galaxy clusters. This instrument will be able to perform all-sky surveys in polarization, allowing us to build a rotation-measure (RM) grid based on an
[...] Read more.
The advent of the Square Kilometer Array (SKA) will have unprecedented impact on the study of magnetic fields in galaxy clusters. This instrument will be able to perform all-sky surveys in polarization, allowing us to build a rotation-measure (RM) grid based on an enormous number of sources. However, it is not always obvious how to extract correct information about the strength and the structure of magnetic fields from the RM grid. The simulations presented here help us to investigate this topic as they consist of full-Stokes idealized (because we did not add thermal noise) images of a pair of galaxy clusters between 950–1760 GHz, i.e., the SKA1-MID band 2. These images include not just cluster-embedded radio sources but also foreground and background discrete radio sources populating the simulated portion of the universe. To study the magnetic fields of the simulated galaxy clusters, we applied the RM synthesis technique on the simulated images and compared the “true” cluster RM values with those inferred from RM synthesis. The accuracy of our methodology is guarantee by the excellent agreement that we observed when we considered only the signal from the background radio sources. The presence of a Faraday screen, foreground, and cluster sources, introduces degeneracies and/or ambiguities that make the interpretation of the results more difficult. Full article
(This article belongs to the Special Issue The Power of Faraday Tomography)
Figures

Figure 1

Open AccessReview The Discovery of the Expansion of the Universe
Galaxies 2018, 6(4), 132; https://doi.org/10.3390/galaxies6040132
Received: 2 November 2018 / Revised: 27 November 2018 / Accepted: 29 November 2018 / Published: 3 December 2018
Viewed by 94 | PDF Full-text (2018 KB) | HTML Full-text | XML Full-text
Abstract
Alexander Friedmann, Carl Wilhelm Wirtz, Vesto Slipher, Knut E. Lundmark, Willem de Sitter, Georges H. Lemaître, and Edwin Hubble all contributed to the discovery of the expansion of the universe. If only two persons are to be ranked as the most important ones
[...] Read more.
Alexander Friedmann, Carl Wilhelm Wirtz, Vesto Slipher, Knut E. Lundmark, Willem de Sitter, Georges H. Lemaître, and Edwin Hubble all contributed to the discovery of the expansion of the universe. If only two persons are to be ranked as the most important ones for the general acceptance of the expansion of the universe, the historical evidence points at Lemaître and Hubble, and the proper answer to the question, “Who discovered the expansion of the universe?”, is Georges H. Lemaître. Full article
Figures

Figure 1

Open AccessArticle Spectroscopic and Photometric Variability of Three Oxygen Rich Post-AGB “Shell” Objects
Galaxies 2018, 6(4), 131; https://doi.org/10.3390/galaxies6040131
Received: 28 June 2018 / Revised: 28 November 2018 / Accepted: 29 November 2018 / Published: 3 December 2018
Viewed by 101 | PDF Full-text (865 KB)
Abstract
Light, color, and radial velocity data (2007–2015) for HD 161796, V887 Her, and HD 331319, three oxygen-rich post-AGB stars, have thus far not provided direct support for the binary hypothesis to explain the shapes of planetary nebulae and severely constrain the properties of
[...] Read more.
Light, color, and radial velocity data (2007–2015) for HD 161796, V887 Her, and HD 331319, three oxygen-rich post-AGB stars, have thus far not provided direct support for the binary hypothesis to explain the shapes of planetary nebulae and severely constrain the properties of any such undetected companions. The light and velocity curves are complex, showing similar periods and variable amplitudes. Nevertheless, over limited time intervals, we compared the phasing of each. The color curves appear to peak with or slightly after the light curves, while the radial velocity curves peak about a quarter of a cycle before the light curves. Thus it appears that these post-AGB stars are brightest when smallest and hottest. The spectra of these objects are highly variable. The H α line has multiple, variable emission and absorption components. In these oxygen-rich post-AGB stars atmospheric lines, such as near-infrared Ca ii triplet and low-excitation atomic lines, also have multiple components and sometimes show line doubling, indicative of shocks induced by pulsation. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
Open AccessArticle The Rate of Short-Duration Gamma-Ray Bursts in the Local Universe
Galaxies 2018, 6(4), 130; https://doi.org/10.3390/galaxies6040130
Received: 31 October 2018 / Revised: 23 November 2018 / Accepted: 27 November 2018 / Published: 30 November 2018
Viewed by 226 | PDF Full-text (2357 KB) | HTML Full-text | XML Full-text
Abstract
Following the faint gamma-ray burst, GRB 170817A, coincident with a gravitational wave-detected binary neutron star merger at d40 Mpc, we consider the constraints on a local population of faint short duration GRBs (defined here broadly as T90<4 s).
[...] Read more.
Following the faint gamma-ray burst, GRB 170817A, coincident with a gravitational wave-detected binary neutron star merger at d 40 Mpc, we consider the constraints on a local population of faint short duration GRBs (defined here broadly as T 90 < 4 s). We review proposed low-redshift short-GRBs and consider statistical limits on a d 200 Mpc population using Swift/Burst Alert Telescope (BAT), Fermi/Gamma-ray Burst Monitor (GBM), and Compton Gamma-Ray Observatory (CGRO) Burst and Transient Source Experiment (BATSE) GRBs. Swift/BAT short-GRBs give an upper limit for the all-sky rate of < 4 y 1 at d < 200 Mpc, corresponding to < 5% of SGRBs. Cross-correlation of selected CGRO/BATSE and Fermi/GBM GRBs with d < 100 Mpc galaxy positions returns a weaker constraint of 12 y 1 . A separate search for correlations due to SGR giant flares in nearby ( d < 11 Mpc) galaxies finds an upper limit of < 3 y 1 . Our analysis suggests that GRB 170817A-like events are likely to be rare in existing SGRB catalogues. The best candidate for an analogue remains GRB 050906, where the Swift/BAT location was consistent with the galaxy IC 0327 at d 132 Mpc. If binary neutron star merger rates are at the high end of current estimates, then our results imply that at most a few percent will be accompanied by detectable gamma-ray flashes in the forthcoming LIGO/Virgo science runs. Full article
Figures

Figure 1

Open AccessArticle Is There a Polarization Horizon?
Galaxies 2018, 6(4), 129; https://doi.org/10.3390/galaxies6040129
Received: 29 October 2018 / Revised: 23 November 2018 / Accepted: 25 November 2018 / Published: 30 November 2018
Viewed by 120 | PDF Full-text (5504 KB) | HTML Full-text | XML Full-text
Abstract
Modern radio spectrometers make measurement of polarized intensity as a function of Faraday depth possible. I investigate the effect of depolarization along a model line of sight. I model sightlines with two components informed by observations: a warm ionized medium with a lognormal
[...] Read more.
Modern radio spectrometers make measurement of polarized intensity as a function of Faraday depth possible. I investigate the effect of depolarization along a model line of sight. I model sightlines with two components informed by observations: a warm ionized medium with a lognormal electron density distribution and a narrow, denser component simulating a spiral arm or Hii region, all with synchrotron-emitting gas mixed in. I then calculate the polarized intensity from 300–1800 MHz and calculate the resulting Faraday depth spectrum. The idealized synthetic observations show far more Faraday complexity than is observed in Global Magneto-Ionic Medium Survey observations. In a model with a very nearby Hii region observed at low frequencies, most of the effects of a “depolarization wall” are evident: the Hii region depolarizes background emission, and less (but not zero) information from beyond the Hii region reaches the observer. In other cases, the effects are not so clear, as significant amounts of information reach the observer even through significant depolarization, and it is not clear that low-frequency observations sample largely different volumes of the interstellar medium than high-frequency observations. The observed Faraday depth can be randomized such that it does not always have any correlation with the true Faraday depth. Full article
(This article belongs to the Special Issue The Power of Faraday Tomography)
Figures

Figure 1

Open AccessArticle The Challenge of Detecting Intracluster Filaments with Faraday Rotation
Galaxies 2018, 6(4), 128; https://doi.org/10.3390/galaxies6040128
Received: 28 September 2018 / Revised: 23 November 2018 / Accepted: 26 November 2018 / Published: 30 November 2018
Viewed by 97 | PDF Full-text (3103 KB) | HTML Full-text | XML Full-text
Abstract
The detection of filaments in the cosmic web will be crucial to distinguish between the possible magnetogenesis scenarios, and future large polarization surveys will be able to shed light on their magnetization level. In this work, we use numerical simulations of galaxy clusters
[...] Read more.
The detection of filaments in the cosmic web will be crucial to distinguish between the possible magnetogenesis scenarios, and future large polarization surveys will be able to shed light on their magnetization level. In this work, we use numerical simulations of galaxy clusters to investigate their possible detection. We compute the Faraday Rotation signal in intracluster filaments and compare it to its surrounding environment. We find that the expected big improvement in sensitivity with the SKA-MID will in principle allow the detection of a large fraction of filaments surrounding galaxy clusters. However, the contamination of the intrinsic Faraday Rotation of background polarized sources will represent a big limiter to the number of objects that can be significantly detected. We discuss possible strategies to minimize this effect and increase the chances of detection of the cosmic web with the large statistics expected from future surveys. Full article
(This article belongs to the Special Issue The Power of Faraday Tomography)
Figures

Figure 1

Open AccessArticle The Extraordinary Linear Polarisation Structure of the Southern Centaurus A Lobe Revealed by ASKAP
Galaxies 2018, 6(4), 127; https://doi.org/10.3390/galaxies6040127
Received: 1 November 2018 / Revised: 26 November 2018 / Accepted: 28 November 2018 / Published: 29 November 2018
Viewed by 113 | PDF Full-text (2104 KB) | HTML Full-text | XML Full-text
Abstract
We present observations of linear polarisation in the southern radio lobe of Centaurus A, conducted during commissioning of the Australian Square Kilometre Array Pathfinder (ASKAP) telescope. We used 16 antennas to observe a 30 square degree region in a single 12-h pointing over
[...] Read more.
We present observations of linear polarisation in the southern radio lobe of Centaurus A, conducted during commissioning of the Australian Square Kilometre Array Pathfinder (ASKAP) telescope. We used 16 antennas to observe a 30 square degree region in a single 12-h pointing over a 240 MHz band centred on 913 MHz. Our observations achieve an angular resolution of 26 × 33 arcseconds (480 parsecs), a maximum recoverable angular scale of 30 arcminutes, and a full-band sensitivity of 85 μ Jy beam 1 . The resulting maps of polarisation and Faraday rotation are amongst the most detailed ever made for radio lobes, with order 10 5 resolution elements covering the source. We describe several as-yet unreported observational features of the lobe, including its detailed peak Faraday depth structure, and intricate networks of depolarised filaments. These results demonstrate the exciting capabilities of ASKAP for widefield radio polarimetry. Full article
(This article belongs to the Special Issue The Power of Faraday Tomography)
Figures

Figure 1

Open AccessArticle Untangling Cosmic Magnetic Fields: Faraday Tomography at Metre Wavelengths with LOFAR
Galaxies 2018, 6(4), 126; https://doi.org/10.3390/galaxies6040126
Received: 26 October 2018 / Revised: 16 November 2018 / Accepted: 19 November 2018 / Published: 29 November 2018
Viewed by 127 | PDF Full-text (1222 KB) | HTML Full-text | XML Full-text
Abstract
The technique of Faraday tomography is a key tool for the study of magnetised plasmas in the new era of broadband radio-polarisation observations. In particular, observations at metre wavelengths provide significantly better Faraday depth accuracies compared to traditional centimetre-wavelength observations. However, the effect
[...] Read more.
The technique of Faraday tomography is a key tool for the study of magnetised plasmas in the new era of broadband radio-polarisation observations. In particular, observations at metre wavelengths provide significantly better Faraday depth accuracies compared to traditional centimetre-wavelength observations. However, the effect of Faraday depolarisation makes the polarised signal very challenging to detect at metre wavelengths (MHz frequencies). In this work, Faraday tomography is used to characterise the Faraday rotation properties of polarised sources found in data from the LOFAR Two-Metre Sky Survey (LoTSS). Of the 76 extragalactic polarised sources analysed here, we find that all host a radio-loud AGN (Active Galactic Nucleus). The majority of the sources (∼64%) are large FRII radio galaxies with a median projected linear size of 710 kpc and median radio luminosity at 144 MHz of 4 × 10 26 W Hz 1 (with ∼13% of all sources having a linear size >1 Mpc). In several cases, both hotspots are detected in polarisation at an angular resolution of ∼20 . One such case allowed a study of intergalactic magnetic fields on scales of 3.4 Mpc. Other detected source types include an FRI radio galaxy and at least eight blazars. Most sources display simple Faraday spectra, but we highlight one blazar that displays a complex Faraday spectrum, with two close peaks in the Faraday dispersion function. Full article
(This article belongs to the Special Issue The Power of Faraday Tomography)
Figures

Figure 1

Open AccessReview The First- and Second-Order Fermi Acceleration Processes in BL Lacertae Objects
Galaxies 2018, 6(4), 125; https://doi.org/10.3390/galaxies6040125
Received: 19 October 2018 / Revised: 15 November 2018 / Accepted: 21 November 2018 / Published: 25 November 2018
Viewed by 167 | PDF Full-text (3738 KB) | HTML Full-text | XML Full-text
Abstract
BL Lacertae objects constitute a rare class of active galactic nuclei with extreme observational features attributed to the Doppler-boosted emission from a relativistic jet, closely aligned to our line-of-sight. Their spectral energy distribution, extending over 17–19 orders of frequency from radio to the
[...] Read more.
BL Lacertae objects constitute a rare class of active galactic nuclei with extreme observational features attributed to the Doppler-boosted emission from a relativistic jet, closely aligned to our line-of-sight. Their spectral energy distribution, extending over 17–19 orders of frequency from radio to the TeV energy range, is of non-thermal origin and shows a typical two-component structure. The lower-energy component, ranging from the radio to X-rays in the high-energy peaked BL Lacertae sources, is widely accepted to be a synchrotron radiation emitted by ultra-relativistic charged particles, to be initially accelerated via the Blandford–Znajek mechanism or magneto-hydrodynamic processes in the vicinity of the central super-massive black hole. However, the accelerated particles should lose the energy, sufficient for the emission of the keV-GeV photons, very quickly and the source can maintain its flaring state on the daily-weekly timescales only if some additional acceleration mechanisms are continuously at work. According to different studies and simulations, the particles can gain tremendous energies due to the propagation of relativistic shocks through the jet: By means of first-order Fermi mechanism at the shock front, or they undergo an efficient stochastic (second-order Fermi) acceleration close to the shock front, in the turbulent jet medium. Our intensive X-ray spectral study of TeV-detected, bright BL Lacertae objects (Mrk 421, 1ES 1959+650, Mrk 501) often show the signatures of the stochastic acceleration, while those related to the first-order Fermi process arefound relatively rarely. The TeV-undetected sources (1H 1516+660, BZB J1341+3959, BZB J1237+6258) mostly do not show the signatures of the efficient stochastic acceleration in their jets. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
Figures

Figure 1

Open AccessConference Report Magnetizing the Cosmic Web during Reionization
Galaxies 2018, 6(4), 124; https://doi.org/10.3390/galaxies6040124
Received: 2 November 2018 / Revised: 20 November 2018 / Accepted: 21 November 2018 / Published: 23 November 2018
Viewed by 197 | PDF Full-text (1165 KB) | HTML Full-text | XML Full-text
Abstract
Increasing evidence suggests that cosmological sheets, filaments, and voids may be substantially magnetized today. The origin of magnetic fields in the intergalactic medium (IGM) is, however, currently uncertain. It seems well known that non-standard extensions to the physics of the standard model can
[...] Read more.
Increasing evidence suggests that cosmological sheets, filaments, and voids may be substantially magnetized today. The origin of magnetic fields in the intergalactic medium (IGM) is, however, currently uncertain. It seems well known that non-standard extensions to the physics of the standard model can provide mechanisms susceptible of magnetizing the universe at large. Perhaps less well known is the fact that standard, classical physics of matter–radiation interactions actually possesses the same potential. We discuss a magnetogenesis mechanism based on the exchange of momentum between hard photons and electrons in an inhomogeneous IGM. Operating in the neighborhood of ionizing sources during the epoch of reionization, this mechanism is capable of generating magnetic seeds of relevant strengths over scales comparable to the distance between ionizing sources. In addition, summing up the contributions of all ionizing sources and taking into account the distribution of gas inhomogeneities, we show that this mechanism leaves the IGM, at the end of reionization, with a level of magnetization that might account, when amplification mechanisms take over, for the magnetic fields strengths in the current cosmic web. Full article
(This article belongs to the Special Issue The Power of Faraday Tomography)
Figures

Figure 1

Open AccessArticle The Astrochemistry Implications of Quantum Chemical Normal Modes Vibrational Analysis
Galaxies 2018, 6(4), 123; https://doi.org/10.3390/galaxies6040123
Received: 8 October 2018 / Revised: 12 November 2018 / Accepted: 20 November 2018 / Published: 23 November 2018
Viewed by 151 | PDF Full-text (298 KB) | HTML Full-text | XML Full-text
Abstract
Understanding the molecular vibrations underlying each of the unknown infrared emission (UIE) bands (such as those found at 3.3, 3.4, 3.5, 6.2, 6.9, 7.7, 11.3, 15.8, 16.4, 18.9 μm) observed in or towards astronomical objects is a vital link to uncover the
[...] Read more.
Understanding the molecular vibrations underlying each of the unknown infrared emission (UIE) bands (such as those found at 3.3, 3.4, 3.5, 6.2, 6.9, 7.7, 11.3, 15.8, 16.4, 18.9 μ m) observed in or towards astronomical objects is a vital link to uncover the molecular identity of their carriers. This is usually done by customary classifications of normal-mode frequencies such as stretching, deformation, rocking, wagging, skeletal mode, etc. A large literature on this subject exists and since 1952 ambiguities in classifications of normal modes via this empirical approach were pointed out by Morino and Kuchitsu New ways of interpretation and analyzing vibrational spectra were sought within the theoretical framework of quantum chemistry. Many of these methods cannot easily be applied to the large, complex molecular systems which are one of the key research interests of astrochemistry. In considering this demand, a simple and new method of analyzing and classifying the normal mode vibrational motions of molecular systems was introduced. This approach is a fully quantitative method of analysis of normal-mode displacement vector matrices and classification of the characteristic frequencies (fundamentals) underlying the observed IR bands. Outcomes of applying such an approach show some overlap with customary empirical classifications, usually at short wavelengths. It provides a quantitative breakdown of a complex vibration (at longer wavelengths) into the contributed fragments such as their aromatic or aliphatic components. In addition, in molecular systems outside the classical models of chemical bonds and structures where the empirical approach cannot be applied, this quantitative method enables an interpretation of vibrational motion(s) underlying the IR bands. As a result, further modifications in the structures (modeling) and the generation of the IR spectra (simulating) of the UIE carriers, initiated by proposing a PAH model, can be implemented in an efficient way. Here fresh results on the vibrational origin of the spectacular UIE bands based on astrochemistry molecular models, explored through the lens of the quantitative method applied to thousands of different vibrational motion matrices are discussed. These results are important in the context of protoplanetary nebulae and planetary nebulae where various molecular species have been uncovered despite their harsh environments. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
Figures

Figure 1

Open AccessReview Very High-Energy Emission from the Direct Vicinity of Rapidly Rotating Black Holes
Galaxies 2018, 6(4), 122; https://doi.org/10.3390/galaxies6040122
Received: 31 October 2018 / Revised: 19 November 2018 / Accepted: 19 November 2018 / Published: 22 November 2018
Viewed by 141 | PDF Full-text (6394 KB) | HTML Full-text | XML Full-text
Abstract
When a black hole accretes plasmas at very low accretion rate, an advection-dominated accretion flow (ADAF) is formed. In an ADAF, relativistic electrons emit soft gamma-rays via Bremsstrahlung. Some MeV photons collide with each other to materialize as electron-positron pairs in the magnetosphere.
[...] Read more.
When a black hole accretes plasmas at very low accretion rate, an advection-dominated accretion flow (ADAF) is formed. In an ADAF, relativistic electrons emit soft gamma-rays via Bremsstrahlung. Some MeV photons collide with each other to materialize as electron-positron pairs in the magnetosphere. Such pairs efficiently screen the electric field along the magnetic field lines, when the accretion rate is typically greater than 0.03–0.3% of the Eddington rate. However, when the accretion rate becomes smaller than this value, the number density of the created pairs becomes less than the rotationally induced Goldreich–Julian density. In such a charge-starved magnetosphere, an electric field arises along the magnetic field lines to accelerate charged leptons into ultra-relativistic energies, leading to an efficient TeV emission via an inverse-Compton (IC) process, spending a portion of the extracted hole’s rotational energy. In this review, we summarize the stationary lepton accelerator models in black hole magnetospheres. We apply the model to super-massive black holes and demonstrate that nearby low-luminosity active galactic nuclei are capable of emitting detectable gamma-rays between 0.1 and 30 TeV with the Cherenkov Telescope Array. Full article
(This article belongs to the Special Issue Radio Galaxies at TeV Energies)
Figures

Figure 1

Open AccessArticle Combining Faraday Tomography and Wavelet Analysis
Galaxies 2018, 6(4), 121; https://doi.org/10.3390/galaxies6040121
Received: 3 October 2018 / Revised: 16 November 2018 / Accepted: 19 November 2018 / Published: 22 November 2018
Viewed by 155 | PDF Full-text (2525 KB) | HTML Full-text | XML Full-text
Abstract
We present a concept for using long-wavelength broadband radio continuum observations of spiral galaxies to isolate magnetic structures that were only previously accessible from short-wavelength observations. The approach is based on combining the RM Synthesis technique with the 2D continuous wavelet transform. Wavelet
[...] Read more.
We present a concept for using long-wavelength broadband radio continuum observations of spiral galaxies to isolate magnetic structures that were only previously accessible from short-wavelength observations. The approach is based on combining the RM Synthesis technique with the 2D continuous wavelet transform. Wavelet analysis helps to isolate and recognize small-scale structures which are produced by Faraday dispersion. We find that these structures can trace galactic magnetic arms as illustrated by the case of the galaxy NGC 6946 observed at λ = 17 22 cm. We support this interpretation through the analysis of a synthetic observation obtained using a realistic model of a galactic magnetic field. Full article
(This article belongs to the Special Issue The Power of Faraday Tomography)
Figures

Figure 1

Open AccessArticle Science Pipelines for the Square Kilometre Array
Galaxies 2018, 6(4), 120; https://doi.org/10.3390/galaxies6040120
Received: 5 October 2018 / Revised: 14 November 2018 / Accepted: 15 November 2018 / Published: 20 November 2018
Viewed by 222 | PDF Full-text (5403 KB) | HTML Full-text | XML Full-text
Abstract
The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and the largest Big Data project in the known Universe. The first phase of the project will generate on the order of five zettabytes of data per year. A
[...] Read more.
The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and the largest Big Data project in the known Universe. The first phase of the project will generate on the order of five zettabytes of data per year. A critical task for the SKA will be its ability to process data for science, which will need to be conducted by science pipelines. Together with polarization data from the LOFAR Multifrequency Snapshot Sky Survey (MSSS), we have been developing a realistic SKA-like science pipeline that can handle the large data volumes generated by LOFAR at 150 MHz. The pipeline uses task-based parallelism to image, detect sources and perform Faraday tomography across the entire LOFAR sky. The project thereby provides a unique opportunity to contribute to the technological development of the SKA telescope, while simultaneously enabling cutting-edge scientific results. In this paper, we provide an update on current efforts to develop a science pipeline that can enable tight constraints on the magnetised large-scale structure of the Universe. Full article
(This article belongs to the Special Issue The Power of Faraday Tomography)
Figures

Figure 1

Open AccessReview Binary Neutron Star and Short Gamma-Ray Burst Simulations in Light of GW170817
Galaxies 2018, 6(4), 119; https://doi.org/10.3390/galaxies6040119
Received: 16 August 2018 / Revised: 9 November 2018 / Accepted: 13 November 2018 / Published: 19 November 2018
Viewed by 158 | PDF Full-text (6724 KB) | HTML Full-text | XML Full-text
Abstract
In the dawn of the multi-messenger era of gravitational wave astronomy, which was marked by the first ever coincident detection of gravitational waves and electromagnetic radiation, it is important to take a step back and consider our current established knowledge. Numerical simulations of
[...] Read more.
In the dawn of the multi-messenger era of gravitational wave astronomy, which was marked by the first ever coincident detection of gravitational waves and electromagnetic radiation, it is important to take a step back and consider our current established knowledge. Numerical simulations of binary neutron star mergers and simulations of short GRB jets must combine efforts to understand such complicated and phenomenologically rich explosions. We review the status of numerical relativity simulations with respect to any jet or magnetized outflow produced after merger. We compare what is known from such simulations with what is used and obtained from short GRB jet simulations propagating through the BNS ejecta. We then review the established facts on this topic, as well as discuss things that need to be revised and further clarified. Full article
Figures

Figure 1

Open AccessArticle Strategy to Explore Magnetized Cosmic Web with Forthcoming Large Surveys of Rotation Measure
Galaxies 2018, 6(4), 118; https://doi.org/10.3390/galaxies6040118
Received: 3 September 2018 / Revised: 13 November 2018 / Accepted: 13 November 2018 / Published: 16 November 2018
Viewed by 144 | PDF Full-text (1208 KB) | HTML Full-text | XML Full-text
Abstract
The warm-hot intergalactic medium (WHIM) is a candidate for the missing baryons in the Universe. If the WHIM is permeated with the intergalactic magnetic field (IGMF), the Faraday rotation measure (RM) of the WHIM is imprinted in linearly-polarized emission from extragalactic objects. In
[...] Read more.
The warm-hot intergalactic medium (WHIM) is a candidate for the missing baryons in the Universe. If the WHIM is permeated with the intergalactic magnetic field (IGMF), the Faraday rotation measure (RM) of the WHIM is imprinted in linearly-polarized emission from extragalactic objects. In this article, we discuss strategies to explore the WHIM’s RM from forthcoming radio broadband and wide-field polarization sky surveys. There will be two observational breakthroughs in the coming decades; the RM grid and Faraday tomography. They will allow us to find ideal RM sources for the study of the IGMF and give us unique information of the WHIM along the line of sight. Full article
(This article belongs to the Special Issue The Power of Faraday Tomography)
Figures

Figure 1

Open AccessFeature PaperReview Fermi: Monitoring the Gamma-Ray Universe
Galaxies 2018, 6(4), 117; https://doi.org/10.3390/galaxies6040117
Received: 17 October 2018 / Revised: 2 November 2018 / Accepted: 6 November 2018 / Published: 16 November 2018
Viewed by 146 | PDF Full-text (6064 KB) | HTML Full-text | XML Full-text
Abstract
Since 2008, the Large Area Telescope and the Gamma-ray Burst Monitor on the Fermi Gamma-ray Space Telescope have been monitoring the entire sky at energies from about 8 keV to more than 1 TeV. Photon-level data and high-level data products are made publicly
[...] Read more.
Since 2008, the Large Area Telescope and the Gamma-ray Burst Monitor on the Fermi Gamma-ray Space Telescope have been monitoring the entire sky at energies from about 8 keV to more than 1 TeV. Photon-level data and high-level data products are made publicly available in near-real time, and efforts continue to improve the response time. This long-duration, all-sky monitoring has enabled a broad range of science, from atmospheric phenomena on Earth to signals from high-redshift sources. The Fermi instrument teams have worked closely with multiwavelength and multi-messenger observers and theorists to maximize the scientific return from the observatory, and they look forward to continued cooperative efforts as Fermi moves into its second decade of operation. Full article
(This article belongs to the Special Issue Monitoring the Non-Thermal Universe)
Figures

Figure 1

Open AccessReview Radio Galaxies at VHE Energies
Galaxies 2018, 6(4), 116; https://doi.org/10.3390/galaxies6040116
Received: 9 October 2018 / Revised: 7 November 2018 / Accepted: 8 November 2018 / Published: 15 November 2018
Viewed by 189 | PDF Full-text (3110 KB) | HTML Full-text | XML Full-text
Abstract
Radio Galaxies have by now emerged as a new γ-ray emitting source class on the extragalactic sky. Given their remarkable observed characteristics, such as unusual gamma-ray spectra or ultrafast VHE variability, they represent unique examples to probe the nature and physics of
[...] Read more.
Radio Galaxies have by now emerged as a new γ-ray emitting source class on the extragalactic sky. Given their remarkable observed characteristics, such as unusual gamma-ray spectra or ultrafast VHE variability, they represent unique examples to probe the nature and physics of active galactic nuclei (AGN) in general. This review provides a compact summary of their observed characteristics at very high γ-ray energies (VHE; greater than 100 GeV) along with a discussion of their possible physics implications. A particular focus is given to a concise overview of fundamental concepts concerning the origin of variable VHE emission, including recent developments in black hole gap physics. Full article
(This article belongs to the Special Issue Radio Galaxies at TeV Energies)
Figures

Figure 1

Open AccessArticle Galactic Density and Evolution Based on the Virial Theorem, Energy Minimization, and Conservation of Angular Momentum
Galaxies 2018, 6(4), 115; https://doi.org/10.3390/galaxies6040115
Received: 25 May 2018 / Revised: 26 August 2018 / Accepted: 9 November 2018 / Published: 15 November 2018
Viewed by 187 | PDF Full-text (4006 KB) | HTML Full-text | XML Full-text
Abstract
Spiral galaxies are spinning, internally densified objects. The Virial Theorem explains galactic rotation curves via its linkage of the rotation rate to the gravitational self-potential (Ug) and the moment of inertia of oblate spheroids. We devise a new analytical solution
[...] Read more.
Spiral galaxies are spinning, internally densified objects. The Virial Theorem explains galactic rotation curves via its linkage of the rotation rate to the gravitational self-potential (Ug) and the moment of inertia of oblate spheroids. We devise a new analytical solution that allows galactic mass and volumetric density (kg m−3) profiles to be extracted from velocity and its derivative as functions of equatorial radius. This inverse model of rotation curves is direct, unambiguous, and parameter-free. To probe galactic evolution, we combine energy minimization, angular momentum conservation, and the Virial Theorem. The characteristic flat shape of spiral galaxies results from an initial vertical collapse of a spinning, colossal molecular cloud, which reduces Ug while conserving angular momentum. Subsequent inward densification further lowers Ug, producing bulges, but conserving angular momentum requires mass loss, achieved by the outward movement of the distal parts of the spiral arms. Many of the evolutionary patterns of spiral galaxies are exhibited by the changing shapes of hurricanes during formation and dissipation. In contrast, elliptical galaxies evolve from a cloud with roughly random orbits into progressively rounder, internally denser objects, with angular momentum conserved by the development of vertically oriented jets. Galactic evolution is governed by the initial inventory of mass and angular momentum, resulting in separate paths for elliptical and spiral galaxies, as is codified in Hubble’s tuning fork diagram. Full article
Figures

Figure 1

Open AccessReview A Review of the Theory of Galactic Winds Driven by Stellar Feedback
Galaxies 2018, 6(4), 114; https://doi.org/10.3390/galaxies6040114
Received: 30 August 2018 / Revised: 25 October 2018 / Accepted: 30 October 2018 / Published: 1 November 2018
Viewed by 266 | PDF Full-text (2465 KB) | HTML Full-text | XML Full-text
Abstract
Galactic winds from star-forming galaxies are crucial to the process of galaxy formation and evolution, regulating star formation, shaping the stellar mass function and the mass-metallicity relation, and enriching the intergalactic medium with metals. Galactic winds associated with stellar feedback may be driven
[...] Read more.
Galactic winds from star-forming galaxies are crucial to the process of galaxy formation and evolution, regulating star formation, shaping the stellar mass function and the mass-metallicity relation, and enriching the intergalactic medium with metals. Galactic winds associated with stellar feedback may be driven by overlapping supernova explosions, radiation pressure of starlight on dust grains, and cosmic rays. Galactic winds are multiphase, the growing observations of emission and absorption of cold molecular, cool atomic, ionized warm and hot outflowing gas in a large number of galaxies have not been completely understood. In this review article, I summarize the possible mechanisms associated with stars to launch galactic winds, and review the multidimensional hydrodynamic, radiation hydrodynamic and magnetohydrodynamic simulations of winds based on various algorithms. I also briefly discuss the theoretical challenges and possible future research directions. Full article
(This article belongs to the Special Issue Theory and Observations of Galactic Outflows)
Figures

Figure 1

Open AccessArticle Planetary Nebulae Shaped by Common Envelope Evolution
Galaxies 2018, 6(4), 113; https://doi.org/10.3390/galaxies6040113
Received: 16 July 2018 / Revised: 29 September 2018 / Accepted: 2 October 2018 / Published: 26 October 2018
Cited by 5 | Viewed by 193 | PDF Full-text (1132 KB) | HTML Full-text | XML Full-text
Abstract
The morphologies of planetary nebula have long been believed to be due to wind shaping processes in which a “fast wind” from the central star impacts a previously ejected envelope. It is assumed that asymmetries existing in the “slow wind” envelope would lead
[...] Read more.
The morphologies of planetary nebula have long been believed to be due to wind shaping processes in which a “fast wind” from the central star impacts a previously ejected envelope. It is assumed that asymmetries existing in the “slow wind” envelope would lead to inertial confinement, shaping the resulting interacting wind flow. We present new results demonstrating the effectiveness of Common Envelope Evolution (CEE) at producing aspherical envelopes which, when impinged upon by a spherical fast stellar wind, produce highly bipolar, jet-like outflows. We have run two simple cases using the output of a single PHANTOM SPH CEE simulation. Our work uses the Adaptive Mesh Refinement code AstroBEAR to track the interaction of the fast wind and CEE ejecta allows us to follow the morphological evolution of the outflow lobes at high resolution in 3-D. Our two models bracket low and high momentum output fast winds. We find the interaction leads to highly collimated bipolar outflows. In addition, the bipolar morphology depends on the fast wind momentum injection rate. With this dependence comes the initiation of significant symmetry breaking between the top and bottom bipolar lobes. Our simulations, though simplified, confirm the long-standing belief that CEE can plan a major role in PPN and PN shaping. These simulations are intended as an initial exploration of the post-CE/PPN flow patterns that can be expected from central source outflows and CE ejecta. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
Figures

Figure 1

Open AccessArticle The Power of Low Frequencies: Faraday Tomography in the Sub-GHz Regime
Galaxies 2018, 6(4), 112; https://doi.org/10.3390/galaxies6040112
Received: 17 September 2018 / Revised: 17 October 2018 / Accepted: 21 October 2018 / Published: 23 October 2018
Viewed by 283 | PDF Full-text (4462 KB) | HTML Full-text | XML Full-text
Abstract
Faraday tomography, the study of the distribution of extended polarized emission by strength of Faraday rotation, is a powerful tool for studying magnetic fields in the interstellar medium of our Galaxy and nearby galaxies. The strong frequency dependence of Faraday rotation results in
[...] Read more.
Faraday tomography, the study of the distribution of extended polarized emission by strength of Faraday rotation, is a powerful tool for studying magnetic fields in the interstellar medium of our Galaxy and nearby galaxies. The strong frequency dependence of Faraday rotation results in very different observational strengths and limitations for different frequency regimes. I discuss the role these effects take in Faraday tomography below 1 GHz, emphasizing the 100–200 MHz band observed by the Low Frequency Array and the Murchison Widefield Array. With that theoretical context, I review recent Faraday tomography results in this frequency regime, and discuss expectations for future observations. Full article
(This article belongs to the Special Issue The Power of Faraday Tomography)
Figures

Figure 1

Open AccessCorrection Correction: Rubin, J.L. Applications of a Particular Four-Dimensional Projective Geometry to Galactic Dynamics. Galaxies 2018, 6, 83
Galaxies 2018, 6(4), 111; https://doi.org/10.3390/galaxies6040111
Received: 9 October 2018 / Accepted: 9 October 2018 / Published: 19 October 2018
Viewed by 182 | PDF Full-text (159 KB) | HTML Full-text | XML Full-text
Abstract
The author wishes to make the following corrections to this paper [1]: [...] Full article
Open AccessArticle Close Binaries and the Abundance Discrepancy Problem in Planetary Nebulae
Galaxies 2018, 6(4), 110; https://doi.org/10.3390/galaxies6040110
Received: 27 July 2018 / Revised: 12 October 2018 / Accepted: 15 October 2018 / Published: 19 October 2018
Viewed by 222 | PDF Full-text (866 KB) | HTML Full-text | XML Full-text
Abstract
Motivated by the recent establishment of a connection between central star binarity and extreme abundance discrepancies in planetary nebulae, we have carried out a spectroscopic survey targeting planetary nebula with binary central stars and previously unmeasured recombination line abundances. We have discovered seven
[...] Read more.
Motivated by the recent establishment of a connection between central star binarity and extreme abundance discrepancies in planetary nebulae, we have carried out a spectroscopic survey targeting planetary nebula with binary central stars and previously unmeasured recombination line abundances. We have discovered seven new extreme abundance discrepancies, confirming that binarity is key to understanding the abundance discrepancy problem. Analysis of all 15 objects with a binary central star and a measured abundance discrepancy suggests a cut-off period of about 1.15 days, below which extreme abundance discrepancies are found. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
Figures

Figure 1

Open AccessArticle The Laboratory Astrophysics Spectroscopy Programme at Imperial College London
Galaxies 2018, 6(4), 109; https://doi.org/10.3390/galaxies6040109
Received: 2 August 2018 / Revised: 2 October 2018 / Accepted: 9 October 2018 / Published: 13 October 2018
Viewed by 305 | PDF Full-text (2743 KB) | HTML Full-text | XML Full-text
Abstract
Accurate atomic parameters, such as transition probabilities, wavelengths, and energy levels, are indispensable for the analysis of stellar spectra and the obtainment of chemical abundances. However, the quantity and quality of the existing data in many cases lie far from the current needs
[...] Read more.
Accurate atomic parameters, such as transition probabilities, wavelengths, and energy levels, are indispensable for the analysis of stellar spectra and the obtainment of chemical abundances. However, the quantity and quality of the existing data in many cases lie far from the current needs of astronomers, creating an acute need for laboratory measurements of matching accuracy and completeness to exploit the full potential of the very expensively acquired astrophysical spectra. The Fourier Transform Spectrometer at Imperial College London works in the vacuum ultraviolet-visible region with a resolution of 2,000,000 at 200 nm. We can acquire calibrated spectra of neutral, singly, and doubly ionized species. We collaborate with the National Institute of Standards and Technology (NIST) and the University of Lund to extend our measurements into the infrared region. The aim of this review is to explain the current capabilities of our experiment in an understandable way to bring the astronomy community closer to the field of laboratory astrophysics and encourage further dialogue between our laboratory and all those astronomers who need accurate atomic data. This exchange of ideas will help us to focus our efforts on the most urgently needed data. Full article
(This article belongs to the Special Issue Atomic and Molecular Data Needs for Astronomy and Astrophysics)
Figures

Figure 1

Open AccessArticle Infrared Observations of the Asymmetric Mass Loss of an AGB Star
Galaxies 2018, 6(4), 108; https://doi.org/10.3390/galaxies6040108
Received: 31 July 2018 / Revised: 9 October 2018 / Accepted: 9 October 2018 / Published: 12 October 2018
Viewed by 234 | PDF Full-text (294 KB) | HTML Full-text | XML Full-text
Abstract
We report on the observations of the circumstellar envelope of the AGB star II Lup in the near- and mid-infrared with the use of direct imaging and interferometric techniques. Our findings indicate that the circumstellar envelope is not spherically symmetric and that the
[...] Read more.
We report on the observations of the circumstellar envelope of the AGB star II Lup in the near- and mid-infrared with the use of direct imaging and interferometric techniques. Our findings indicate that the circumstellar envelope is not spherically symmetric and that the majority of the emission originates within 0.5 arcsec from the star. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
Figures

Figure 1

Open AccessArticle Testing the Anomalous Growth of the Black Hole Radius from AGN
Galaxies 2018, 6(4), 107; https://doi.org/10.3390/galaxies6040107
Received: 18 July 2018 / Revised: 17 September 2018 / Accepted: 20 September 2018 / Published: 3 October 2018
Viewed by 246 | PDF Full-text (393 KB) | HTML Full-text | XML Full-text
Abstract
We analyze constraints on the anomalous growth of the black hole radius or the black hole spin from the X-rays spectrum data of Active Galactic Nuclei (AGN) in NGC 5506. The anomalous growth of the mass or of the spin of a black
[...] Read more.
We analyze constraints on the anomalous growth of the black hole radius or the black hole spin from the X-rays spectrum data of Active Galactic Nuclei (AGN) in NGC 5506. The anomalous growth of the mass or of the spin of a black hole may be unveiled within the framework of models of alternative gravity, including f ( R ) -gravity. Our phenomenological analysis is based on an effective parametrization for the black hole Kerr metric, which is inspired by the antievaporating solutions discovered by Nojiri and Odintsov. We find tight constraints on the parameter space of anomalous metrics. Intriguingly, we find that a more than secularly growing solution can better fit current data. Our result opens a pathway towards a new phenomenological approach for testing predictions of general relativity and alternative theories of gravity. Full article
Figures

Figure 1

Back to Top